1. Field of the Invention
The invention generally relates to analyte collection and is useful, for example, in analyte detection and analysis systems and methods. In particular, the invention concerns preconcentrators, which are used in a wide variety of collection and analysis systems, and the use of metal organic framework molecules as sorbents in preconcentrators or similar devices, including particularly portable micropreconcentrators.
2. Related Art
A preconcentrator is a device that is used to collect a sample for analysis of particular constituents in the sample called analytes. A preconcentrator is particularly useful in detecting analytes that are present in very low concentrations in a sample. For example, a preconcentrator may be used in combination with a detection device to collect a sample, thereby increasing its concentration, before transferring it to a detector for analysis of analytes of interest. As a low-concentration sample gas containing a mixture of compounds passes through a preconcentrator, compounds are captured and sorbed by the preconcentrator over time. During preconcentration, the concentration of the analyte may increase by up to over 1000 times the original concentration of the analyte in the sample. The captured compounds are then desorbed, passed out of the preconcentrator, and conducted to the detector for analysis.
For example, a preconcentrator may be used with a gas chromatograph (GC), a vital instrument used to analyze complex compound mixtures in a variety of environments including clinical, aerospace, military, process control, and other applications. A preconcentrator is required in high performance GC systems because the resolving power of the GC column or the sensitivity of the sensor is often limited by the low ppb (part-per-billion) concentrations of analytes with a wide range of volatility. When connected to a GC, a discrete sample is first captured in the preconcentrator and then thermally desorbed to a polymer-coated separation column in the GC. The sample is then eluted down the column under a positive pressure of an inert carrier gas to the detector.
Separation of the components of the sample by differential partitioning along the column, which is typically ramped to an elevated temperature range during the analysis, permits the identification and quantification of the components of the sample by their retention times and response profiles. The separated components are subsequently detected and recorded in a detector. Preconcentration increases the sensitivity of the detector by concentrating dilute samples that would normally not be detected.
Capillary-tube preconcentrators are conventionally used in GCs. Such preconcentrators include a stainless-steel or glass capillary tube packed with one or more granular sorbent materials. These preconcentrators are typically large in size and have significant power requirements, and as a result, the GC systems they are used with are not portable. This makes conventional GCs practically impossible to use in number of settings including subways, airports, and buildings where analysis of air samples must be performed on-site. These preconcentrators also suffer from large dead volume and limited heating efficiency due to their large thermal mass. In addition, conventional GC systems often have a long analysis cycle and require a large sampling volume. This makes them impractical for use in circumstances in which only a small sample is available for testing and rapid analysis is necessary.
Many efforts have gone into the development of portable micropreconcentrators that exhibit sufficiently high performance to be used in portable GC systems. So far, these portable systems have also been relatively large in size or weight (several kg) and have a large power requirement (tens-to-hundreds of Watts). Further, conventional micropreconcentrators have been unable to raise the concentration of dilute samples to a detectable range so that samples can be analyzed accurately. Thus, there are no commercially available fast preconcentrators that are small enough to fit into a microelectromechanical (MEMS) scale gas analysis system.
Common sorbents used in preconcentrators such as activated carbon, Tenax®, zeolites, carbon nanotubes, Carbopack®, Carbotrap®, Carbosieve®, Carboxen®, Chromosorb®, HayeSep®, silica gel, and glass beads also have several long-standing problems that make them unsuitable or undesirable for use in micropreconcentrators. For example, these sorbents do not have high enough sorption capacity or are not selective enough for specific analytes or and they cause low or incomplete desorption of analyte. In particular, it is known that many toxic gases decompose before they are desorbed from carbon-based sorbents. DB5 is another sorbent in which analytes interfere with each other and make detection difficult. In addition, these sorbents often have slow or incomplete desorption, which makes it very difficult to get accurate concentration readings.
Accordingly, there is a need for improved preconcentrators and, in particular, commercially viable micropreconcentrators, as well as sorbents that provide high gains in concentration of analytes, selective sorption of analytes, allow rapid and complete desorption of analytes, and have high sorption capacities. Further, the sorbent should have thermal stability of to temperatures of up to 300° C. to 400° C. and should not interfere with the sample gas or its analytes. In addition, micropreconcentrators should be compact in size so that they can be used with a portable analytical instrument and require only a small sample volume.